ESTRO 2023 - Abstract Book

S824

Monday 15 May 2023

ESTRO 2023

Results The DVH analysis of all 3 plans for the 10 patients is shown in table 2. Plans optimized with DO show better PTV coverage, lower PTV Dmax and doses to all other organs and also generated less MU. Plan quality improves when ITV-PTV is overridden with higher lung density. The results of robust evaluation (with DO undone) are also shown in table 2. Plans with no DO and DO of 0.15 always meet the ITV coverage requirements while 2 plans with DO of 0.26 fail. For patient 1, 2 scenarios fail to meet the ITV requirement of V90% and for patient 3, 4 scenarios fail at both ITV V95% and V90%. Both patients have very small target volume and low average HU in ITV-PTV area. In the worst case scenario, the Dmax of the PTV, of a ring at 2cm distance and the most critical organ are always lower in plans optimized with DO.

Conclusion The use of DO between ITV-PTV area improves VMAT lung SBRT plan quality in patient with very low lung density. Robust evaluation can be used (without DO) to validate the robustness of the plan. In general, the default lung density value of 0.26 will results in robust plan with superior plan quality. But in cases with very small ITV and low average HU in the ITV PTV margin area, the use of lower lung density of 0.15 is recommended. The use of DO reduces OAR doses and the risk of over dosage caused by setup uncertainties. PD-0976 A novel anatomical robust optimization strategy for prostate cancer: a RADIOSA trial spin-off study M.G. Vincini 1 , F. Pansini 2 , M. Zaffaroni 1 , G. Corrao 1 , F. Cattani 2 , G. Marvaso 1,3 , B.A. Jereczek-Fossa 1,3 1 IEO, European Institute of Oncology IRCCS, Division of Radiation Oncology, Milan, Italy; 2 IEO, European Institute of Oncology IRCCS, Unit of Medical Physics, Milan, Italy; 3 University of Milan, Department of Oncology and Hematoncology, Milan, Italy Purpose or Objective SBRT treatment delivers a relatively high irradiation dose in a limited number of fractions and in a highly conformal manner, resulting in steep dose gradients between targets and OARs. Therefore, geometrical and anatomical variations can have a major impact on the planned dose distribution. Classical optimization considers uncertainties in patient setup, neglecting interfractional variations. Aim of the present study, side project of phase II randomized clinical trial RADIOSA (NCT03940235), is the development and testing of an anatomical robust optimization (aRO) method that can meet target coverage and dosimetric constraints despite variation in PTV position and bladder/rectal filling. Materials and Methods A total of 12 PCa oligometastatic pts with lymph node metastases enrolled within the RADIOSA trial, treated with 30Gy/3fx SBRT at the European Institute of Oncology and with isodose curve 50% intersecting bladder or/and rectum were considered.